1. Field of the Invention
The present invention relates to a front-and-rear wheel drive vehicle in which one of pairs of front and rear wheels is driven by an engine, while the other pair is driven by start assisting motors.
2. Description of the Related Art
The applicant of this patent application has already proposed a front-and-rear wheel drive vehicle as described above in which when the front wheels slip when they are driven by the engine to start the vehicle, the start assisting motors are driven and the dog clutch provided between the motors and the rear wheels is engaged so as to drive the rear wheels to thereby provide a four-wheel drive condition (refer to Japanese Patent Application No.Hei.9-79348).
When a front-and-rear wheel drive vehicle such as described above starts by driving the front wheels with the engine, since the rear wheels follow by virtue of friction force received from the road surface, the movable dog (a dog connected to the rear wheels and motors) of the dog clutch disposed between the motors and rear wheels rotates and there is generated a relative rotation between the stationary dog (a dog fixed to the casing of the transmission) and the movable dog. Even if the dog clutch is tried to be engaged in this state, since the movable dog is repelled by the stationary dog, it is difficult to have a smooth engagement. Conventionally, to cope with this, the motors are driven idly prior to the engagement of the dog clutch to stop thereby the rotation of the movable dog which is rotated by the rear wheels, and then the two dogs are engaged together in a state in which the relative rotation between the stationary and movable dogs is eliminated or reduced.
However, it was difficult to stop the movable dog by running the motors idly, and it was not possible to avoid an engagement shock that is produced when the dog clutch is engaged.
In general, since there are provided on the vehicle wheel speed sensors for detecting the wheel speeds of the rear wheels, the revolution speed of the movable dog can be computed based on the wheel speeds so detected by the sensors. Therefore, if the motors are driven at a speed corresponding to the detected wheel speed of the rear wheels, it is possible to stop the rotation of the movable dog to have a smooth engagement of the dog clutch. However, in order to perform a feedback control so that the motors run at a predetermined motor rotation number, a motor rotation number sensor is required to detect the speed of the motor, and this leads to an increase in cost. It is, therefore, desirable to prevent the engagement shock of the dog clutch without provision of motor rotation number sensors.